The central maxim of astrobiology is “Follow the water”. So far, we have spent billions on Mars probes that have looked for life directly or indirectly by looking for signs of liquid water on Mars.
So shouldn't we look for liquid water on the Moon as well?
Let's review: the main fact that cannot be debated is that there are large zones within the Moon where the temperature and pressure are within the liquid water area of the water phase diagram. Moreover, the thermal gradient of the regolith has been directly measured by instruments emplaced by Apollo 15 and Apollo 17. The measured thermal gradient of ~1.5 K/m is huge compared to the thermal gradient of the underlying basalt (~0.01 to 0.02 K/m--these figures represent a lower bound since they are for solid basalt; to the extent that the underlying bedrock is actually an extensively fractured, blocky "megaregolith", then the thermal gradient will be higher than for solid rock).
The relation between the thermal gradient, heat flow and thermal conductivity is given by:
dT/dz = q/k
dT/dz is the thermal gradient (K/m)
k is the thermal conductivity (W/mK)
q is the heat flow (W/m2)
Given this formula, it is easy to calculate to a first order approximation where the habitable zone in relation to the surface of the Moon should be. So we know with a reasonable scientific certainty that the order of magnitude zones where the temperatures and pressures that favor the existence of liquid water begin, and that they may be found anywhere from within ~10 m to ~1000 m of the surface, depending heavily on the depth of the regolith and the local heat flow.
However, just because the temperature and pressure are right for liquid water does not entail that there is in fact liquid water there. And without a doubt, it is the old fashioned, commonplace perception that the Moon is as dry as a bone that has led the Moon to be largely passed over as an astrobiology science target. However, there is some empirical evidence to support the idea that liquid water may exist in places within the Moon:
First, and most importantly, is the recent Brown University discovery of water at the ppt level within mantle specks encapsulated within glass beads found within the famous orange soil discovered by Cernan and Schmitt of Apollo 17. This concentration of water is comparable to that found within mid-oceanic ridge basalts (MORBs) found on Earth. Which in hindsight is not surprising, considering that the Moon and Earth share a common origin. Moreover, such water deep within the mantle should cause serpentinization of olivine, which would provide both a heat source (since serpentinization is highly exothermic) and an energy source for methanogenic, Lunar lithotrophes in the form of H2. This primordial water on the Moon can provide a continuing source of fresh water and fresh food that could keep a Lunar ecosystem going for billions of years.
Second, there is indirect evidence of liquid water within the Moon in the form of the rimless, oddly-shaped craters. According to Dr. Spudis and others, one likely explanation for these features (which have also been observed on Mercury) is that they were caused by violent, volatile gas expulsions. Water is a common Lunar volatile, and so water in all it's phases is likely to be involved: as steam heated by local hot spots, radioactive decay, and/or serpentinization becomes overpressured and moves toward the surface, as it got to the cold surface layers, it would condense and then freeze. If this layer occurred within the regolith (because of local heating and locally deep regolith), said water would freeze and turn unconsolidated regolith into a rock hard, impervious cement. Then eventually, as heating of liquid water by serpentinization outpaces the conductivity of the overlying non-cemented regolith and it's ability to radiate the heat away into space, the water would turn into a gas, and the pore spaces would become extremely overpressured. The pressure would only increase until something gave way: a violent explosion ensues, and the rimless pit is formed. Note that such events cannot be blamed on water and volatiles leftover from comet impacts.
In addition, we know for a fact that in between the present Lunar epoch, where the Lunar exosphere is a virtual vacuum and molecules move around ballistically, and the Hadean epoch, where the Moon had a 100 Bar rock vapor atmosphere with a temperature in excess of 1500 K, there must have been a second, intermediate epoch (during which occurred the Late Heavy Bombardment) in which the atmospheric pressure varied wildly between periods where a surface boundary exosphere (SBE) prevailed (which is the case in the present epoch), and other periods when the atmospheric pressure almost certainly was well above the triple point of water. As Vondrak mathematically demonstrated, once the atmosphere gets thick enough to cross a very low threshold, upper layers protect lower layers, the solar wind gets deflected around the planet, and the atmosphere becomes stable on a time scale of 103 years in the sense that any given molecule will not get dissipated into space for a period on the order of 103 years (as opposed to the current conditions, where gases are lost to space on a time scale measured in months).
What the weather was like back then is anybody's guess at this point. It would certainly make an interesting numerical climatic modeling project. However, it almost certainly must have been the case that the temperature at times must have been between freezing and boiling and that cometary impacts must have occassionally delivered enough water to cause precipitation; after all, this is during the time when most of the Earth's water is thought to have been delivered via cometary impacts.
Now, it is certainly the case that the Martian meteorite, ALH84001, cannot be taken as conclusive evidence for the existence of life on Mars. On the other hand, ALH84001, and others of its class have conclusively demonstrated that (a) material is exchanged between Mars, the Earth, and the Moon, and (b) that these transfer events can happen at low enough temperatures so that any life contained within such rocks would not be sterilized. Therefore, given the evidence and theory we have available to us today, it is certainly not implausible that life could be transferred from the Earth to the Moon and Mars, or vice versa. Therefore, it is certainly not implausible that any Martian or Earthly life that was transferred to the Moon and happened to land in a little, warm crater lake would have survived. From there, such life forms could make their way into the Lunar groundwater on time scales measured in hours or weeks. Once life got down past the first few meters, they would be home free in the inevitable event that the lake desiccated. Once underground, some of the life would then eventually come into contact with groundwaters fed in part by primordial water moving up from deeper within the mantle and lower crust. The fractures within the megaregolith would provide plenty of living space for microbial life, and the fresh water moving up from below would provide a more or less constant trickle of fresh food in the form of H2 coming from the serpentinization of olivine.
Or alternatively, during this second epoch on the Moon, the Moon was a much wetter, more volcanically active place than it is today: therefore, it is certainly not implausible that there existed Lunar analogues for most of the environments on the early Earth, such as hydrothermal vents, that are thought to be conducive for the spontaneous origin of life. Certainly, it is the case that the Moon did not have as much time as the Earth did for abiogenesis to occur; however, we cannot say with certainty that the Moon did not have enough time for abiogenesis to occur. Indeed, since the Moon cooled faster than the Earth after the end of the Hadean, we cannot say for sure that life did not originate on the Moon first, and was then spread to Planet Earth!
In sum, the case that (1) life could have originated or arrived on the Moon early in its history, that (2) Lunar life might very well still be extant, and that (3) it might very well be found at depths that could be accessed by robots or humans relatively easily, cannot be reasonably dismissed out of hand. The reason that Lunar astrobiology has not been discussed much in the past is because the entire case presented here hinges on the very recent announcement in May of 2011 that the Moon contains Earth-equivalent amounts of water within the Moon's mantle (cf. Scientists detect Earth-equivalent amount of water in the moon). Without a plausible, indigenous source of fresh water and energy it would indeed be hard to make a case for life in the Moon. But now that such a source has been found, it is now difficult--once one thoughtfully considers the evidence with an open mind--to presume that life could not exist on the Moon!
Therefore, why not attempt to seek out liquid water deposits on future Lunar missions??
Well, is it the case that it is more likely that life exists on Mars? Perhaps. If we were to assign personal Bayesian prior probabilities to the likelihood that life will eventually be found on the Moon and/or Mars, no doubt Mars would score higher on average. Of course, the real probabilities are either 1 or 0. Either life is present on the Mars and/or the Moon, or it is not. The fact is that we don't know, and we will never know for sure unless we go try and find out.
One thing is certain, however: and that is that the cost and difficulty of Lunar operations is far less than Martian operations at this point in our technological and economic history. Therefore, one is well within reason to question the wisdom of blowing off the Moon pretty much entirely as President Obama would have us do, in order that every spare dollar be spent on a push to Mars in an attempt to find life there. Indeed, wouldn't we all feel kind of stupid if we went to Mars first, only to find out years later that ET was right under our nose on the Moon the whole time?
This is the danger: if we attempt to go straight to Mars first without building up a serious cis-Lunar infrastructure that includes Lunar ISRU propellants, then it's pretty much going to have to be a sort of Apollo-on-steroids, Mars Direct-style sort of architecture that gets us there, where very many tonnes go up into space and very few tonnes come back and little to nothing is reused. Such an architecture is not sustainable in the long term; once "success" was declared, cancelation would be the likely result, and we would be back where we are now--basically at square one.
On the other hand, if we are patient and husband our resources by building permanent infrastructure that accumulates over time, starting with a permanently manned Lunar research station, the new view of the Moon demonstrates we can still do intensive astrobiology with actual human scientists at the same time--albeit on the Moon. If the analysis presented here has no irreparable major holes in it (and I see none so far), then it is the case that we stand a fairly good chance of achieving positive astrobiological results on the Moon for a fraction of the cost and a fraction of the waiting time that a similar Mars effort would entail. And even if the search for liquid water eventually proved futile, or that any liquid water found turned out to be entirely sterile, looking for liquid water on the Moon would not be a waste of resources since the experience gained and equipment developed would prove invaluable to future Mars efforts that will certainly occur sooner or later.